With the recent discovery of high-efficiency thermoelectric (TE) materials, potential applications of TE technology have attracted worldwide interest. TE devices can be used for cooling and power generation purposes in a variety of applications and have the potential for high reliability, long life and environmentally safe operation. While most of the work in thermoelectric technology has focused on the development of new materials, of equal importance is investigation of the fabrication issues regarding incorporation of the newly-developed materials into TE devices.
Filled skutterudites are prospective high-efficiency materials for TE power generation by TE devices having a hot side temperature (TH) of between 450 degrees C. and 600 degrees C. For simplicity and proof of concept, binary n-type and p-type CoSb3 skutterudites are used to fabricate the n-type and p-type legs of thermoelectric devices. Copper is used as the electrode material at the cold side of the device.
Because of the relatively high TH at the hot side of the device, selection of the high-temperature electrode material is important. First, the high-temperature electrode material should neither react with CoSb3 nor diffuse into the CoSb3 at the TH. Second, the high-temperature electrode material should have high electrical and thermal conductivity values. Third, the material should have a thermal expansion coefficient which is comparable to that of CoSb3 to prevent breakage or cracking. Finally, the material should not be oxidized easily.
Due to its high electrical conductivity (18.1 106 Ω−1 m−1) and thermal conductivity (138 W/mK), molybdenum (Mo) is a good candidate for the high temperature electrode material. In addition, its room temperature thermal expansion coefficient is close to that of CoSb3. The room temperature thermal expansion coefficients for both n-type and p-type CoSb3 are about 8.0×10−6 K−1. Furthermore, Mo does not oxidize easily. However, because it has a high melting point (2623 degrees C.), Mo is difficult to be directly joined to CoSb3, which has a melting point of 876 degrees C.
Therefore, utilization of a titanium buffer layer between the molybdenum high-temperature electrode and the CoSb3 n-type and p-type legs is needed in the fabrication of a thermoelectric device since titanium has relatively large electrical and thermal conductivities, a thermal expansion coefficient which is comparable to that of CoSb3, is oxidation-resistant and has a melting point which is much lower than that of molybdenum.